The one rule is lasers-only. Yes, from the start, humans have had laser guns for decades! The tricky part is getting the aliums to put on the vests, but I think the smoothest play is to just present it as "a get together with friends" or "interspecies outreach". If that goes well, *then* you ask (or are asked!) about an actual date...
This might not work if the aliums are legitimately hostile, but at least that gets us X-Piratez. ...In most ways a rather dire world, but plenty of xenodating!
A serious approach... Gosh, I don't know. The XCOM setting is extremely arbitrary, requiring a lot of "we just don't know" or "a space-wizard did it" to work. I find myself thinking of the video game Fate of the World, where essentially the UN has to convince/cajole/bribe the nations of the world into cooperating against the mundane but real threats we face as a species. Trying to end war, poverty, climate change and peak oil. A game both darkly humorous and somewhat hopeful, if cynical.
That makes me think that a "realistic" XCOM experience would be like the lore we see in X-Piratez or X-COM Files, and probably Terra Invicta. Using infiltration and terror to destabilize our world governments until they *choose* to become vassals.
The most effective methods to resist colonialism are outside my knowledge, but it IS something we've done quite a bit of research on in recent history. Maybe we can at least negotiate a less-shitty deal.
I would rush lasers of course, and drone technology! Drones might sound like a security risk, but aliums seem to have NO understanding of electronic warfare, only psionic. Perhaps they don't even know what radio is! Hence heavy weapon platforms (HWPs) draw plasma fire away from our brave soldiers, they're completely immune to enemy control.
Managing the populace is a complicated topic and I'm out of time, so I'd uh... IDK... invent a fake pandemic with lockdowns
(JFC I feel dirty even making that joke XD)